FIG. 12 is a sectional view of conventional acoustic wave device 501 disclosed in Japanese Patent Laid-Open Publication, No. 2003-209458. Acoustic wave device 501 includes piezoelectric substrate 1 made of lithium niobate, interdigital electrode 2 provided on piezoelectric substrate 1, and dielectric layer 3 made of SiO2 covering interdigital electrode 2.
In conventional acoustic wave device 501, a phase velocity of acoustic wave propagating on a surface of piezoelectric substrate 1 is smaller than a phase velocity of slow transverse waves propagating in piezoelectric substrate 1 in order to suppress leakage of the acoustic wave propagating along the interfacial boundary between dielectric layer 3 and piezoelectric substrate 1. It thus becomes necessary for this reason to increase substantially a film thickness of dielectric layer 3. On the other hand, however, an electromechanical coupling coefficient of leaky surface waves decreases as the film thickness increases, and consequently, makes it difficult to broaden the bandwidth.
In addition, the conventional acoustic wave device 501 produces Rayleigh waves due to a low electromechanical coupling coefficient, which causes spurious.